Over the past several decades, muffler designs aiming at compactness and light weight have been introduced in order to accommodate the demands of modern vehicle designs. Being primarily directed to use with four-stroke engines in automobiles and motorcycles, prior art muffler designs have been focused on reducing the size of the muffler system and for enhancing engine efficiency by maintaining low back pressure while adequately reducing exhaust noise by different means. In these designs, the exhaust pipe is partially or wholly enclosed within the body of the muffler to accommodate a duct shaped in a “jellyroll” or spiral passageway enclosed in an outer shell comprising the muffler housing. The spiral passageway is of reduced cross section relative to the header pipe in which exhaust gases increase in velocity and reduced pressure in a gradual manner, thereby greatly reducing the noise associated with the expansion of these gases, while maintaining low pressure and forward flow within the muffler so as greatly lower the backpressure on the engine. Examples of such designs are described in U.S. Pat. No. 3,066,755 to Diehl, U.S. Pat. No. 3,692,142 to Stemp, U.S. Pat. No. 3,927,731 to Lancaster. Later, more elaborate methods of noise abatement were combined with the spiral flow passage, as exemplified in U.S. Pat. No. 4,579,195 to Nieri and U.S. Pat. No. 5,612,006 to Fisk.
More recently, streamlining muffler systems for two-stroke engines has been addressed. For a two-stroke engine, backpressure is an issue, but in the opposite sense in relation to four-stroke engines, and efforts have been made to design an exhaust system to maintain a certain level of backpressure so that the air/fuel mixture does not empty too quickly from the cylinder on the down-stroke of the piston. The quintessential exhaust processing system for a two-stroke engine has been the tuned straight pipe, adding to the passive backpressure control of the air/fuel charge in the cylinder by sending positive pressure pulses to the cylinder synchronized with the down-stroke to push fresh un-combusted air/fuel charge that had escaped into the exhaust system back into the cylinder just before the compression/combustion stroke of the piston. While the straight tuned pipe works well to enhance two-stroke engine efficiency, and reduce exhaust system noise, in the case of small vehicles and hand tools powered by small two-stroke engines, tuned pipes are in many instances longer and bulkier than the very vehicle or devices on which they are mounted, adding significant weight as well. For instance, radio controlled model aircraft are hampered by the presence of a tuned pipe exhaust because it is difficult to hide or streamline the tuned pipe for increasing aerodynamic efficiency. U.S. Pat. No. 6,684,633 to Jett addresses this issue and describes a compact muffler designed for radio controlled aircraft and small engine-powered tools. U.S. Pat. No. 6,959,782 to Brower et al. describes a muffler design based on similar principles for two-cylinder two-stroke motorcycle engines. Both designs comprise compact “tuned” exhaust systems, whereby the tuned exhaust pipe is rolled into a spiral passage leading to an expansion chamber. These mufflers are claimed to be effectively tuned pipes folded into a compact form factor, however fall short of a perfect tuned pipe exhaust because of the combination of the abrupt angles along the folded course of flow, and the rectangular cross section of the spiral passage itself, effectively frustrate the propagation of pressure waves, greatly detuning the system.